This invention relates generally to the detection of ions in mass spectrometry, and more particularly to a data acquisition system including methods of operation and apparatus for determining ion abundances at pre-selected time intervals of one or more ionic spectra.
The science of mass spectrometry has been proven to be a valuable tool in analytical chemistry. Mass spectrometry is premised on the fact that electrically neutral molecules of a sample can be charged or ionized and their motion controlled by electric and magnetic fields. The response of a charged molecule to magnetic and electric fields is influenced by the mass-to-charge ratio of the ion so that ions of a specific mass-to-charge ratio can be selectively detected.
Mass spectrometers differ from each other primarily in the way in which ions of different mass-to-charge ratios are distinguished from each other. Magnetic sector mass spectrometers separate ions of equal energy by the ions' momentum as they are reflected or dispersed in a magnetic field. Quadrapole mass spectrometers separate ions based upon their rate of acceleration in response to a high frequency radio frequency field in the presence of a direct current field. Ion cyclotrons and ion trap mass spectrometers discriminate ions on the frequency or dimensions of their resonant oscillations in alternating current fields. Time-of-flight mass spectrometers discriminate ions according to their velocity over a fixed distance.
Although relatively straightforward in design, time-of-flight (hereinafter "TOF") mass spectrometers produce data at a very high rate. Because ions having different mass-to-charge ratios may be present in a single sample, they will strike the detectors at different times according to their velocity or kinetic energy. The detector output signal comprises a sequence of ion arrival responses which are compressed within a very short time interval, generally less than one-tenth of a microsecond. Within a hundred microseconds, all of the ions, including the heaviest, have traveled the length of the TOF spectrometer and arrived at the detector to produce a spectrum of this sample molecule. Up to as many as one million spectra may be produced for a given sample analyzed. Additionally, these spectra may need to be separated into chronologically ordered sets. The time scale would be on the order of one millisecond.
Only a small segment containing certain ionic compounds of all of the data produced by the analysis of a given sample may be of interest. In the past, however, scientists had to collect data over the entire spectra produced by the sample. To reduce the amount of data produced, and to focus in on the ionic compound of interest, it has been proposed to turn the detection circuit on just prior to the predicted arrival time or window of a selected compound. Details of such a system are disclosed in U.S. Pat. No. 5,367,162, owned by the assignee of the invention. This patent also provides a thorough discussion of the prior art and its disclosure is incorporated herein by reference. However, none of the prior devices are capable of continuous and uninterrupted detection, collection, and processing of time-of-flight spectra. More specifically, none of the prior art devices detect and continuously convert the analog signals to digital signals for selection, summation, and processing using a compact system operating at a substantially reduced power level than heretofore achieved.